1. Field of the Invention
This invention relates to an epoxy resin composition and an epoxy-resin encapsulated semiconductor device encapsulated with the epoxy resin composition. In particular, this invention is for providing an epoxy resin composition for encapsulating a semiconductor which shows flame retardance without a flame retardant such as a halogenated organic compound and antimony and may give an improved productivity because of its higher moldability, as well as an epoxy-resin encapsulated semiconductor device which is heat-resistant during soldering.
2. Description of the Related Art
Recent market trend to size reduction, weight saving and higher performance in electronic devices has led to more integrated semiconductors and accelerated surface mounting of a semiconductor package. Meanwhile, as business activity giving consideration to the global environment has become more significant, there have been attempts to shifting from a consumeristic society to a cyclical society. Thus, material design allowing for segregation and recovery has become significant. Waste Electrical and Electronic Equipment (WEEE) and Restriction of Certain Hazardous Substances (RoHS) established by EU (European Union) ordain that due to its harm, the use of lead which is a solder component should be completely abolished by 2006 except certain applications, and that in terms of a flame retardant, halogenated flame retardants predominantly used should be segregated for recovery.
However, a lead-free solder has a higher melting point than a conventional lead/tin solder. It may lead to elevation of a temperature during mounting by soldering such as infrared reflow and solder immersion from a current level, 220 to 240° C. to 240 to 260° C. Such elevation in a mounting temperature may increase cracks in a resin during mounting, making it difficult to ensure the reliability of the semiconductor device. Furthermore, in terms of a lead frame, the use of a lead frame plated with nickel/palladium instead of external solder plating has been investigated in the light of the requirement of deleading in external solder plating. A nickel/palladium plating has a poor adhesion property to a common encapsulating material, leading to delamination in an interface during mounting and cracks in a resin. Particularly, since thin packages such as TSOP and LQFP have been mainly used, ensuring reliability in a semiconductor device has become a significant challenge, when using such a new material.
For solving these problems, for example, Japanese Laid-open Patent Publication No. 1994-80763 has proposed that for improving heat resistance in a solder, a biphenyl epoxy resin and a phenol aralkyl hardener are used to form a low water-absorbing epoxy resin composition comprising a filler with a higher density. Such a composition has not, however, met the above strict requirement. Furthermore, for example, Japanese Laid-open Patent Publication Nos. 1997-3167, 1997-3161 and 1997-235353 have proposed a low water-absorbing epoxy resin composition comprising a phenol aralkyl or naphthol aralkyl resin having a biphenylene structure and have disclosed that it can improve heat resistance in a solder. Any of these cannot, however, meet the requirement for flame retardance without a halogenated organic compound or antimony as a flame retardant. Since segregation for recovery is needed from environmental consideration when using such a flame retardant, there has been room for improvement in achieving adequate flame retardance.
As materials providing excellent flame retardance without a halogenated organic compound or antimony, there have been disclosed an epoxy resin composition as a combination of a Novolac type phenolic resin comprising a biphenyl derivative and/or naphthalene derivative and a Novolac type epoxy resin comprising a biphenyl derivative and/or naphthalene derivative (for example, see Japanese Laid-open Patent Publication No. 1999-140277). Such an epoxy resin composition may be effective for improving flame retardance because its molecular structure has many aromatic rings which promote carbonization during burning and its crosslinking density is so low that low-boiling components generated by thermal decomposition during burning cause foaming, which forms a surface oxygen-blocking layer. Furthermore, since cured products from such epoxy resins are adequately water-unabsorbable and heat-resistant during soldering, they have been marketed and used in various applications. The epoxy resin composition can improve heat resistance during soldering owing to its low elasticity and low water-absorption. However, it gives a soft and lipophilic mold after curing. Thus, its defective moldability may cause reduction in the productivity in continuous production, which is a problem to be solved.
There is, therefore, still the need for an epoxy resin composition for encapsulating a semiconductor, which can provide a good productivity and exhibit excellent heat resistance during soldering, taking environmental effects into account.